CAMBRIDGE, MASS.
— It's a sultry day on the gritty industrial fringe of the Massachusetts Institute of Technology campus, and as pedestrians stroll by, none of them even bother to glance at a dome that looks like a city water tank.

It's not. But if the public mistakes a nuclear reactor for a water tank, that's fine with John Bernard, director of the MIT Nuclear Reactor Laboratory. Less attention is better. He knows, however, that neither he nor federal authorities can assume a terrorist is so inattentive.

So Dr. Bernard, a beefy man with close-cropped hair, has studied a theoretical airliner crash into the MIT containment dome, a feat that would require great flying skills, given that the structure is only a few stories tall. An engine might get through the shell, the study showed.

Bernard has also examined a hypothetical truck-bomb explosion. "The truck-bomb scenario is more likely, more realistic," he says. "But honestly, with two feet of steel-reinforced concrete, even that isn't going to bother us."

Perhaps so. But after years of being virtually ignored as a security threat because of their relatively small amounts of nuclear fuel, the nation's 26 university research reactors  and hundreds of other research reactors worldwide (see story)  are being fingered by nuclear security experts as prime terrorist targets.

The result is a real educational dilemma for universities with nuclear-engineering programs: Can campus reactors be outfitted with enough security to thwart terrorists and still make economic sense? With such heavy guarding, can colleges avoid creating the impression that they're on the verge of war?

"Nobody wants to talk about university research reactors," says George Bunn, an architect of arms control and a nuclear consultant to the Center for International Security and Cooperation at Stanford University. "But they're a big problem because, typically, research reactors are much closer to city populations. The one at the University of Wisconsin is in the middle of Madison. The one at MIT is on a city street."

Big nuclear power plants like the one at Three Mile Island in Middletown, Pa., were the focal point after Sept. 11 because of their large quantities of nuclear fuel. Now  because of doubts about the preparedness of campus police forces, the bomb-grade nuclear fuel in some of the reactors, and the reactors' proximity to large populations  US campus nuclear programs are under a magnifying lens.

Theft and sabotage are the critical threats. Theft of highly enriched weapons-grade uranium (HEU) fuel, like that used at MIT and five other US research reactors, could enable terrorists to build a nuclear weapon, says a new report by Dr. Bunn. Another recent report, by Harvard University's Project on Managing the Atom, says the 300 or so HEU-fueled research reactors in the US and worldwide "remain dangerously insecure."

Commercial US power reactors and most research reactors in the US do not use bomb-grade uranium. Low-enriched uranium (LEU) is the fuel of choice.

Yet since Sept. 11, some experts say that terrorists might try to use conventional explosives to blow up an LEU reactor in a city or on campus, creating a "dirty bomb" that spreads radioactivity and panic. Spent fuel could be stolen for the same purpose.

Proximity to people

Unlike the one at MIT, most university reactors in the US do not even have a protective containment dome. Many are housed in ordinary lab buildings. But it is their proximity to masses of people that is the big concern.

Giant commercial power reactors like the notorious Three Mile Island plant are by law isolated from people by an uninhabited exclusion zone. That zone greatly lessens the threat from radioactive release, because the intensity of radiation diminishes dramatically each mile from the source.

Most university research reactors, however, are located on or near heavily populated campuses or in urban centers. The MIT reactor sits in Cambridge, Mass., one of the most heavily populated areas in the country, with about 15,000 people per square mile.

"Research reactors pose a safety threat because they are not as closely regulated as nuclear power plants," says the US Department of Energy's 2003 congressional budget request. "If attacked with a conventional explosive, some could have a radiological release equivalent to Chernobyl."

This latter vulnerability was driven home last month by the arrest of Jose Padilla, who authorities say was conspiring to create a dirty bomb with materials acquired from "university facilities."

Edwin Lyman is president of the Nuclear Control Institute in Washington, one of a small cadre of watchdog groups that have argued for years that the US Nuclear Regulatory Commission (NRC) needs to boost security requirements for campus reactors.

"The vulnerability of research reactors to sabotage is a very sensitive issue," Mr. Lyman says. "It's so sensitive to the NRC that almost no public information about their internal analysis has been revealed. Many of these university reactors are poorly secured and they're in the middle of highly populated areas."

Efforts to boost security

NRC spokesman Victor Dricks denies that "nonpower" reactors  a category that includes university research reactors  are weak on security. But he adds that improvements are being made.

"We are looking at other steps that could be taken to strengthen security at the nonpower reactors," Mr. Dricks says. "These reactors, by and large, have very small quantities of radioactive materials on site, and the danger they pose is markedly lower than large commercial power reactors  and the security is commensurate with the risk."

Lyman says commercial nuclear plants are required to have security forces trained in armed response to repel small numbers of armed attackers  three, to be precise. "Research reactors don't have even that requirement. They're not required to be protected against sabotage," he says.

That may change. Like their commercial brethren, university reactor operators were advised last month by the NRC to further tighten security. The MIT reactor reported publicly that it had hired an armed guard to be on site 24 hours a day. And campus police and Cambridge police are on the alert. But some wonder if that is enough.

"From the beginning, university reactors were thought of as innocent," part of the Atoms for Peace program, Bunn says. "If you look at current NRC rules, the focus is still all on [commercial] power reactors."

"We have issued advisories and licenses that have enhanced and heightened their security posture above and beyond normal," he says. "We believe they have an adequate level of security."

Security experts also agree that US research reactors are somewhat less of a target than they used to be. Since the mid-1980s, the NRC has required US research reactors  and others overseas that use US-supplied nuclear fuel  to convert to LEU fuel, which cannot easily be used to make a nuclear bomb. Eight US research reactors, however, including MIT's, still use HEU fuel.

Regardless of the type of fuel, their main vulnerability is sabotage or theft, Lyman and others say.

In the 1980s, Daniel Hirsch had his students at the University of California, Los Angeles, track a shipment of spent fuel from the campus reactor back to its source. At one point, the drivers left the truck full of radioactive spent fuel unattended for hours in a parking lot.

"I hope things have gotten better, but I'm not optimistic," says Mr. Hirsch, now executive director of the Committee to Bridge the Gap, a nuclear watchdog group in Los Angeles."When you put this stuff in the hands of university people who are not used to heavy security, or even trucking companies, it's a bit like the Keystone Cops."

Dwindling ranks of nuclear engineers

Such security concerns, combined with a public-funding crunch in higher education, have arrived at a bad time for nuclear-engineering schools. Some nuclear reactors are on the brink of being shut down. It's also bad for the nuclear-power industry  which has not had an order for a new power plant in decades  and for nuclear-weapons maintenance and nuclear-waste disposal projects, all of which require well-trained people.

"We risk losing a critical mass of knowledge," says Ralph Butler, interim director and chief operating officer of the University of Missouri research reactor program in Columbia. "There are so few new folks coming in that university administrators, with the declining economy, have just been eliminating programs."

Undergraduate and master's degree enrollments nationally slumped 3 percent a year from 1980 to 1992, and have fallen 15 percent annually since then, according to Kenneth Roberts, a former commissioner of the NRC.

Some universities began shuttering their reactors and closing nuclear engineering departments in the late 1960s. The number of such reactors has fallen from more than 60 two decades ago to 26 today. Cornell University and the University of Michigan are still officially on the NRC's list, but are in the process of pulling the plug on their research reactors. Even MIT is weighing that step.

Most observers agree that the decline has been caused by a lack of student demand and the cost of running the reactors. At MIT, the nation's largest program, there are about 100 graduate students and 20 undergraduates.

Inside a reactor's control room

At MIT, William Kennedy, a fifth-year student who is certified by the NRC to operate the reactor, monitors gauges and switches, making sure the reaction in the room above is running smoothly. Safety features on the reactor are supposed to shut it off automatically if problems arise.

"There's a shortage of engineers in the nuclear industry," he says. "So I'm not too worried about finding a job when I graduate. It's a great career and I'm excited about it."

But there are too few like Mr. Kennedy. Working alongside him is Andrew Baughns, another senior. A mathematics major, he too is licensed by the NRC, but for him this is just a well-paying campus job, not a career.

"The campus in general is laid back, but here we're pretty security conscious  and this building is pretty secure against truck bombs," he says. "It's not easy to get our fuel. They'd all die in the process and not accomplish anything."

Rising security concerns add to cost  and cost is a factor in whether reactors like this one live or die, observers say. The MIT reactor gets hundreds of thousands of dollars to pay for its own operations from grants and from fees for irradiating materials for medicinal and other purposes. So do the University of Missouri reactor and others. Still, MIT spent $840,000 on the reactor last year, says Dr. Bernard, the program's director.

On the plus side, MIT's reactor is one of only a few university reactors to have a containment dome. But protection against terrorism is still an issue.

Some information about the reactor, including its location and proximity to major streets and a rail line, is easily observable and available on the Internet, if not widely known by the public.

Since last year, MIT has removed from the Internet some detailed floor plans and diagrams of the reactor building.

The level of concern about MIT as a potential target varies. "The MIT nuclear reactor does not pose any terrorist threat," city fire officials reported at a Cambridge round table on security issues last October. "It contains some low-level radioactive material in neither the quantity nor quality to be used as a weapon."

Even though it is the nation's second most powerful university research reactor, MIT's has only about 1/600 the power of a commercial reactor. The water that cools its gauzy blue radioactive core is only as hot as a dishwasher's. Such factors have put many people at ease.

But others say that if terrorists stole fuel from the reactor, they might have enough to make a nuclear weapon.

A Hiroshima-type bomb, the simplest construction, would require at least 50 kilograms (110 pounds) of the kind of fuel that MIT uses  bomb-grade highly enriched uranium (U-235). A published report says as little as 12 kilograms may be needed for a different design. And some say a bomb could be made with even less.

At least 9 kilograms, nearly 20 pounds, of U-235 are inside the MIT reactor at any one time  a fact posted on the International Atomic Energy Agency website.

Fresh fuel is not stockpiled on site, Bernard says.

But spent fuel, which accumulates on site, retains most of its lethal bomb-grade qualities, although it is so radioactive that it would be difficult for a terrorist to transport. "We try to keep our spent [fuel] and fresh fuel at zero, so even if someone got in, there would be nothing to steal." Bernard says.

Despite the threat of truck bombs, budget battles, and the struggle to sell enough nuclear medicine to stay in business, Bernard soldiers on. "I think we can survive this. I hope we can. If we can't, then terrorists have succeeded in destroying something very important."

German reactor raises concerns

A new German research reactor that is fueled with highly enriched uranium is raising questions among those concerned about nuclear proliferation.

The German government is poised to grant an operating license to an HEU-fueled FRMM-II research reactor at the Technical University of Munich. The reactor would use about 40 kilograms (88 pounds) of HEU each year, supplied by Russia, says Alexander Glaser, a graduate student at the Massachusetts Institute of Technology. It would establish a large supply chain vulnerable to theft, and set a "horrible precedent," he says.

Highly enriched uranium is the perfect fuel for research reactors. And the same scientific specifications make it ideal for building a simple atom bomb like the one that destroyed Hiroshima.

For terrorists, a key issue is acquiring enough HEU to build a bomb. Recent reports warn that time is running out to gather this material, located in 345 civilian reactors in more than 58 countries.

Most HEU is controlled by national armed forces. But about 20 tons is in civilian hands, fueling reactors, medicine, and industry. (Ten to 20 kilograms is needed to build an atomic bomb.)

Around 1986, the United States began to gather up HEU and promote the conversion of research reactors to low-enriched uranium (LEU), which can't be easily used in a bomb.

Alan Kuperman, a policy analyst with the Nuclear Control Institute in Washington, says the US program, called the Reduced Enrichment for Research and Test Reactors, is "one of the unsung heroes" of nuclear nonproliferation efforts. It is run by the Department of Energy for about $5 million annually.

A few years later, the International Atomic Energy Agency got on board. At least 34 of 71 reactors in 19 countries were converted, says Mr. Glaser. Yet that work is being threatened. "If Germany operates a new reactor with HEU fuel, South Africa refuses to convert its reactor to available LEU fuel.... [A] resurgence of HEU commerce could soon follow," Kuperman wrote in a January report.

Part of the problem, he and Glaser say, is that the Germans are able to justify using the HEU by charging that the US is dragging its feet on converting six university reactors "in process" (see list) and pointing to research reactors at MIT and at the University of Missouri at Columbia that have no plans to convert.

The director of the MIT reactor, John Bernard, says he is happy to oversee a conversion to LEU fuel once a blend usable in the reactor is available. "We'll be doing that as soon as possible," he says.